A monochrometer is an optical instrument that can select a narrow band of wavelengths of light from a source which contains a broader spectrum. Spectrometers are the combination of a monochrometer and a detector such that the output of a spectrometer is an electrical signal which is proportional to the intensity of light in the selected narrow band. Monochrometers and spectrometers are used in many important commercial and defense applications, some of which include chemical analysis by optical absorption, emission line characterization, thin film thickness analysis, and optical characterization of mirrors and filters.
The optical properties of an unknown material can reveal important information leading to a determination of its composition or physical properties. For instance, spectral analysis of optical emission lines are used to determine the atomic species of gaseous material. A second example is the routine use of optical spectra by the semiconductor industry to determine the thickness of multilayer thin films. These measurements are made with instruments incorporating optical spectrometers. A typical spectrometer is a precision instrument that usually consists of an entrance slit, a prism or grating, a couple of mirrors or lenses, and an exit slit. Lenses would normally be used to focus the light into the entrance slit and from the exit slit onto a detector. To scan through the spectrum, the grating or prism is rotated mechanically. The grating or prism separates the light into its spectral components and these are selected by the exit slit and measured with an optical detector.
The conventional optical spectrometer is a large, expensive, precision instrument. Its quality is characterized by its ability to separate spectral components or in other words, by its resolution. Analytical equipment that incorporates optical spectrometers are by nature expensive and therefore relegated to applications that can justify the expense. While current spectrometers perform their function well, broader application of optical measurement techniques would be achieved with a small and less expensive alternative.
The present invention relates to a miniature optical spectrometer and methods for manufacturing and using such an instrument. The process takes advantage of microfabrication techniques to produce a microspectrometer that incorporates a wavelength selective micromechanical component and an optical detector. Microspectrometers offer significant advantages over existing instruments including significantly smaller size, lower cost, faster data acquisition rate, and much greater reliability. Because of these advantages, much broader application of optical measurement techniques can be achieved. The microspectrometer can also be built as a multisensor to measure fluid composition, pressure, mass loading transients and microscale turbulent properties of fluids. In these applications variations in the incoming optical signal from a light source are measured and correlated with the selected property or physical characteristic of the fluid being analyzed.
The microspectrometer consists of a mechanical bridge structure which is fabricated on a substrate. The bridge contains a region near its center in which an optical mirror is placed. The mirror is designed to be reflective over a broad range of wavelengths and is fabricated using standard optical thin film deposition techniques or techniques used in conventional microfabrication technology. The bridge extends over the substrate material upon which a second mirror with the same spectral response has been fabricated. The mirror on the bridge and the mirror on the substrate are separated by air, an inert gas, a fluid, or a vacuum in the gap. The combination of the two mirrors and the gap create a miniature Fabry-Perot cavity. Providing an optical cavity where two mirrors are positioned adjacent to one another creates a spacing or gap such that at least one of the mirrors become transmissive over a narrow band of wavelengths. The band over which the mirrors become transmissive depends upon the spacing and the refractive index of the material, if any, located within the gap.
The Fabry-Perot cavity therefore acts as an interference filter which permits the transmission of a narrow band of wavelengths as determined by the quality of the mirrors and the width of the gap. If the gap width is varied, the center frequency for the transmitted light also varies. Moving the bridge relative to the substrate varies the gap between the bridge and the substrate, thus changing the frequency of the transmitted light.
In a further enhancement, a detector can be placed between the lower mirror and the substrate. The detector would be a photosensitive structure with sensitivity in the spectral region transmitted by the mirrors. It could be configured into a photoconductive or photovoltaic sensor with its output proportional to the intensity of the light transmitted by the Fabry-Perot cavity. Certain preferred embodiments employ a charge coupled device (CCD) as a detector.
A preferred embodiment of the spectrometer includes a means of moving the bridge relative to the substrate. One technique would be to incorporate electrostatic force plates. They can be fabricated in a transparent conductive material and be part of the lower mirror structure or can be separate and to the sides of the lower mirror structure. In the latter case, the bridge length must be sufficient to accommodate the force plates. If an electric field is applied between the force plates and the bridge, a resultant force is produced in the bridge which pulls the bridge toward the substrate. This force is roughly proportional to the square of the applied electric field. These force plates can be used to move the bridge in a controlled manner over a range equal to about ⅓ of the total gap between the force plate and the bridge. Motion beyond this point results in unstable behavior which tends to pull the bridge down to the force plates suddenly. To be safe, the motion of the bridge should be restricted to a value less than ⅓ of the gap for static DC operation. If an AC field is applied to the force plates through a series capacitor, it is not necessary to restrict the range of motion to ⅓ the gap spacing, thereby permitting larger controlled motions of the bridge. In a dynamic sense, the bridge can be made to resonate at one of its resonant frequencies by applying a time varying electric field with a frequency equivalent to that of the resonant frequency of the bridge. By making use of resonance, the bridge could be operated over greater excursions with a lower applied field.
The position of the bridge relative to the substrate or in other words, the gap spacing controls the wavelengths of the light transmitted into the detector. It is therefore important to monitor the bridge to substrate spacing. This can be accomplished by using a capacitive detection technique. A set of electrodes is placed under the bridge and the capacitance between the electrodes and the bridge is measured. It is inversely proportional to the gap spacing. This measurement can be made using a number of electronic techniques that include electronic bridge circuits, oscillators and switched capacitor circuits.
In use, a light source consisting of a range of wavelengths whose distribution and amplitudes are to be determined is introduced to the spectrophotometer from the top of the bridge. The bridge is excited into resonance by the application of an electric field. The selected wavelength of the Fabry-Perot cavity varies in time synchronously with the bridge motion. The position of the bridge is monitored with the position detectors. This output along with the output from the detector provides all the information needed to determine the spectral distribution.
This bridge positioning and detection subsystem also has non optical sensor applications. As discussed in a later section, it has all of the hardware required for a microscale force balance system. With modified electronics, and use of a diaphragm bridge, the microspectrometer can be extended into a multisensor capable of measuring local mechanical and electric forces in the media which is being optically monitored.